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Thread: Mstp Wiring

  1. #1
    Join Date
    Nov 2005
    Location
    Shreveport, Louisiana
    Posts
    2,329

    Mstp Wiring

    OK I am interested from my experiences if anyone has a theory or facts as why:
    JCI uses 3 wire for FC Bus (MSTP).
    And Allerton and Delta use 2 wires for the MSTP trunk.

    Before you ask I have only worked on Delta, Allerton and JCI.

    Is there some internal grounding going on in 2 wire boxes and JCI uses 3 , well actually 4 if you include the shield,to ground between controllers.

    Does JCI use 3 because thats is the standard for N2, and they just kept it that way?

    I am looking for professional opions, thoughts, experiences, solutions and not another manufacturer bashing thread.

    Thank you in advance for your cander and professionalism.
    UA Local 141

  2. #2
    Join Date
    Dec 2004
    Location
    Texas
    Posts
    16
    BACnet MSTP is an implementation of the RS485 standard. There is a wealth of information available on the web if you search that.

    I would recommend reading the article and the comments posted on Peter Chipkin's website:http://www.chipkin.com/articles/rs48...two-wire-rs485. I believe Peter occasionally posts here.

    I would also add that in my experience that a well grounded shield can also help a system survive the voltage induced by a nearby lightning strike.

    Ira

  3. #3
    Join Date
    Apr 2008
    Location
    Louisiana
    Posts
    232
    N2 devices can talk without the reference wire hooked up. Try it. I "think" that the reference wire is used as a drain/ground. I used to do installations for the local JCI branch and we never used the shield at all. Just cut it off and wrapped with tape to cover the foil. Delta recommends that you ground their controllers and install terminators on both end of the loop, but I don't find this necessary smaller comm loops. Alerton is prob the same.

  4. #4
    Join Date
    Mar 2006
    Location
    Chicagoland
    Posts
    44
    Some of the proprietary RS-485 based protocols have been based on a two wire concept. The RS-485 is a differential signal which is presented on two wires. Internally the communication power supply is isolated from any supply whose common can be tied to <earth>ground. There is a circuit to help keep the signal range within the limits of the RS-485 tranceiver. The Shield pin is used to help provide noise immunity, and must at least be A.C. coupled to earth to be effective.

    The MS/TP spec. does call for a third wire 'reference' that gets tied to the common of each communication circuit's power supply. The reference is not a shield; it should not be tied to earth ground, and should be connected at each end.
    The reference line normally will not provide immunity, although a manufacturer could certainly A.C. couple the reference to earth to provide some noise rejection. '3 wire' MS/TP connections should probably use the 1.5 TSP cabling that provides a reference and a shield.

    Some manufacturers have spent a lot of effort engineering their 2 wire RS-485 implementations into robust, reliable interfaces. MS/TP may run fine using them, but there may be some complications when 2 wire and 3 wire interfaces are connected on the same trunk segment.

  5. #5
    Join Date
    May 2002
    Posts
    9,564
    Quote Originally Posted by ira fuse View Post
    BACnet MSTP is an implementation of the RS485 standard. There is a wealth of information available on the web if you search that.

    I would recommend reading the article and the comments posted on Peter Chipkin's website:http://www.chipkin.com/articles/rs48...two-wire-rs485. I believe Peter occasionally posts here.

    I would also add that in my experience that a well grounded shield can also help a system survive the voltage induced by a nearby lightning strike.

    Ira
    They seem to have some decent information on bacnet MSTP but you can trash what little they write about LON. It's one thing to bias, but another to provide false information. Mr Cosby provides false information and really needs to learn about the protocol before authoring an article.

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